[Jack Gilbert, the faculty director at the University of Chicago’s Microbiome Center and a professor of surgery] has done extensive research on how the trillions of microbes in our guts digest food, and the look on his face when I told him about the all-beef diet was unamused. He began rattling off the expected ramifications: “Your body would start to have severe dysregulation, within six months, of the majority of the processes that deal with metabolism; you would have no short-chain fatty acids in your cells; most of the by-products of gastrointestinal polysaccharide fermentation would shut down, so you wouldn’t be able to regulate your hormone levels; you’d enter into cardiac issues due to alterations in cell receptors; your microbiota would just be devastated.”

While much of the internet has been following this story in a somewhat snide way, Gilbert appeared genuinely concerned and saddened: “If she does not die of colon cancer or some other severe cardiometabolic disease, the life—I can’t imagine.”

“A company [in Japan] there created a man-po-kei, a pedometer. And man stands for ‘10,000,’ po stands for ‘step,’ and kei stands for ‘meter’ or ‘gauge.’” Ten thousand, it turns out, “is a very auspicious number” in Japanese culture, said Theodore Bestor, a Harvard researcher of Japanese society and culture, in an email. “That is, it seems likely to me that the 10,000 steps goal was subsidiary to having a good-sounding name for marketing purposes.” Whatever the reason for the adoption of this particular number, “It resonated with people at the time, and they went man-po-kei-ing all over the place,” said Tudor-Locke.

Pubmed indexes 175 articles published since June 2015, the date of the linked article above, containing the phrase "10,000 steps". And as weird as its origin story is, the recommendation has been validated, more or less, and has gone on to be endorsed fairly universally.